Minor tweaking of index special-space definitions so that the various

index types can be reliably distinguished by examining the special space
on an index page.  Per my earlier proposal, plus the realization that
there's no need for btree's vacuum cycle ID to cycle through every possible
16-bit value.  Restricting its range a little costs nearly nothing and
eliminates the possibility of collisions.
Memo to self: remember to make bitmap indexes play along with this scheme,
assuming that patch ever gets accepted.

1 files changed, 15 insertions, 10 deletions

diff --git a/src/include/access/gist.h b/src/include/access/gist.h
index 52d3a2fe1a0..eab6a01bfd5 100644
--- a/src/include/access/gist.h
+++ b/src/include/access/gist.h
@@ -9,7 +9,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/gist.h,v 1.57 2007/01/05 22:19:50 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/access/gist.h,v 1.58 2007/04/09 22:04:05 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -56,26 +56,31 @@
 /*
  * Page opaque data in a GiST index page.
  */
-#define F_LEAF			(1 << 0)
-#define F_DELETED		(1 << 1)
+#define F_LEAF				(1 << 0)
+#define F_DELETED			(1 << 1)
 #define F_TUPLES_DELETED	(1 << 2)
 
 typedef XLogRecPtr GistNSN;
 
 typedef struct GISTPageOpaqueData
 {
-	uint32		flags;			/* 29 bits are unused for now */
-	BlockNumber rightlink;
-
-	/*
-	 * the only meaning - change this value if page split.
-	 */
-	GistNSN		nsn;
+	GistNSN		nsn;			/* this value must change on page split */
+	BlockNumber rightlink;		/* next page if any */
+	uint16		flags;			/* see bit definitions above */
+	uint16		gist_page_id;	/* for identification of GiST indexes */
 } GISTPageOpaqueData;
 
 typedef GISTPageOpaqueData *GISTPageOpaque;
 
 /*
+ * The page ID is for the convenience of pg_filedump and similar utilities,
+ * which otherwise would have a hard time telling pages of different index
+ * types apart.  It should be the last 2 bytes on the page.  This is more or
+ * less "free" due to alignment considerations.
+ */
+#define GIST_PAGE_ID		0xFF81
+
+/*
  * This is the Split Vector to be returned by the PickSplit method.
  * PickSplit should check spl_(r|l)datum_exists. If it is 'true',
  * that corresponding spl_(r|l)datum already defined and